fuzz: add missing overrides to signature_checker

Summary:

and also
- add missing parentheses in fuzz/scriptnum_ops.cpp
- remove useless unsigned int conditional in fuzz/script.cpp

These changes fix 5 compile warnings in gcc 10.

Backport of core PR19548.

This is a bit pointless, since for now there is no GCC 10 version for
AFL and the fuzzer sanitizer is not supported. I added another fix to
prevent a -Werror=missing-braces warning in deserialize.cpp.

Test Plan:
With GCC (so no fuzzer enabled):

ninja bitcoin-fuzzers

Check there is no warning.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9133